Referring to FIGS. 1 through 3 for illustrating the conventional cylinder type screen printing machine, a cylinder driving gear 2 is carried on a support shaft of a suction cylinder 1, said driving gear 2 meshing with an oscillably mounted sector gear 3. The sector gear 3 may be swung reciprocably by rotation of a crank lever 5 through a crank rod 6 so that the suction cylinder 1 may be rotated in the fore and aft directions within a certain travel stroke. A pair of rack driving gears 7 are mounted on both end portions lengthwise of the suction cylinder 1.
A rack frame 10 is mounted movably in the fore and aft directions by slide bearings 9 at the four corners thereof for passage of a pair of slide guide shafts which in turn are mounted on the top of the suction cylinder 1. A pair of toothed racks 11 are mounted on both sides of the rack frame 10 for meshing with the rack driving gears 7. A screen plate 12 is mounted in a screen plate frame 13 which in turn is mounted in the rack frame 10. The rack frame 10 may be moved reciprocally in the fore and aft directions along the slide guide shaft 8 with forward and reverse rotations of the rack driving gears 7 caused by forward and reverse rotations of the suction cylinder 1.
Printing paper sheets P are fed one by one to a point A below the screen plate 12 by a paper sheet feed mechanism 14 and advanced therefrom to a point B along the outer peripheral surface of the suction cylinder 1, as they are gripped by a grip pawl 15 carried by the suction cylinder 1. The sheets P are held by suction on the suction cylinder 1 between said points A and B. The sheets P are released at point B by the grip pawl 15. At this time, the suction force so far holding the paper sheet P about the periphery of the cylinder 1 is also released, and the paper sheets P are discharged by a paper take-out device 16.
Printing takes place during the feed motion as described above. Thus, when the paper sheet P is gripped by the claw 15 and about to pass through an apex point C of the suction cylinder 1, a squeegee 17 mounted on top of the screen plate 12 is lowered for pressing the screen plate 12 for printing the sheet P passing therebelow, said screen plate being in its forward stroke timed with forward rotation of the suction cylinder 1. At this time, a doctor 18 mounted on the screen plate 12 in a side-by-side relation to the squeegee 17 is in its raised position.
After discharge of the printed sheet P, the suction cylinder 1 is rotated in the reverse direction and, in timing therewith, the screen plate 12 is moved rearward. At this time, the squeegee 17 is raised, while the doctor 18 is lowered for smoothing the ink applied to the printing plate 12.
As shown in FIG. 3, the squeegee 17 and the doctor 18 are mounted on one side of the screen plate 12 by way of a vertically movable squeegee post 20 and a vertically movable doctor post 21 the fore and aft end parts of a centrally fulcrumed lever 19 pivotally supported at its central portion. A cam disc 22 consisting of a portion 22a of a lesser radius of curvature and a portion 22b of a larger radius of curvature is mounted on the crank shaft 4. With rotation of the cam disc 22, the end parts of the lever 19 may be vertically reciprocally moved through a link mechanism consisting of a cam roller 24 on a cam lever 23, a connecting rod 25 and an intermediate lever 26, for alternately raising and lowering the posts 20, 21.
On one side of and parallel to the squeegee post 20, a rack post 27 is mounted vertically movably and carries a toothed rack 28 at its lower end for meshing with a pinion 29. A speed reducing unit 30 connected to a reversible electric motor is connected via a chain 31 to the pinion 29 for vertically reciprocating the rack post 27. A tension spring 32 is placed between the lower end of rack post 27 and the upper portion of the squeegee post 20 for normally biasing the squeegee post 20 in a downward direction.
Thus, during printing, the squeegee 17 is pressed strongly onto screen plate 12 through the squeegee post 20 and by virtue of resiliency of the tension spring 32.
When the lesser radius of curvature portion 22a of the cam disc 22 fronts to the cam roller 24, the squeegee post 20 is lowered under the urging force of spring 32 to lower the squeegee 17, whilst the doctor post 21 is raised to raise the doctor 18. The printing pressure exerted by the squeegee 17 is a function of the spring force of the tension spring 32.
When the larger radius of curvature portion 22b of the cam disc 22 fronts to cam roller 24, the squeegee 17 is raised against the force of the spring 32, whilst the doctor 18 is lowered.
When the rack post 27 is raised by the operation of the speed reducing unit 30, the upper end of the rack post 27 abuts on an engaging member 33 secured to the upper portion of the squeegee post 20 for raising the post 20, the squeegee 17 being also raised slightly. With the squeegee thus raised, the screen plate frame 13 may be dismounted for cleaning the screen plate 12, or again mounted in position, as the occasion may demand.
With the conventional device described above, the squeegee post 20 is mounted to one end extremity of the lever 19, and hence the squeegee 17 cannot be raised well above the screen plate 12. Neither the length of the lever 19 nor the distance between the center of rotation of the lever 19 and the lower end of the squeegee post 20 may be in excess of a certain value in consideration of possible interference with other components of the printing machine and thus a certain limitation is necessarily placed on the upward stroke distance of the squeegee 17.
In addition, in the above conventional device, the pressure applied by the squeegee 17 on the screen plate 12 is a function of the spring force of the tension spring 32. Thus, at a higher printing speed, impacts caused by oscillation of the tension spring 32 affects the squeegee 17 especially at the start of pressing of the squeegee, that is, at the start of printing, thus producing undulating printing irregularities on the printing surface.